The present invention pertains generally to devices and methods that are useful for separating particles of a multi-species plasma according to their mass-charge ratios. More particularly, the present invention pertains to plasma mass filters which operate at plasma densities that are below the collisional density of the multi-species plasma being processed. The present invention is particularly, but not exclusively, useful as a filter for separating and segregating charged particles from a multi-species plasma into more than two different parts.
There are many reasons why it may be desirable to separate a composite material into its constituent elements. Just as there are many such reasons, there are many ways or methods by which this can be accomplished. For one, it is well known that some composite or combination materials can be mechanically separated by means such as sieves, sorters and; diverters. Further, it is known that chemical processes are often useful for separating composites into their separate parts. It happens, however, that some composite materials are extremely difficult to process and, therefore, do not readily lend themselves to the more conventional methods of processing. In particular, nuclear waste is such a composite material.
Recently, efforts have been made to process materials by first vaporizing them, and then causing the vaporized constituent elements to separate from each other. One such process involves the use of a plasma centrifuge. In a plasma centrifuge, the charged particles of a plasma are caused to rotate around a common axis, and to collide with each other as they rotate. As a consequence of these collisions, the heavier mass particles move farther away from the axis of rotation than do the lighter mass particles. Accordingly, the particles are separated according to their respective masses. More recently, however, plasma filters have been developed which rely on physical principles that are much different than those relied on by plasma centrifuges.
An example of a plasma filter and its methods of operation are provided in U.S. Pat. No. 6,096,220, issued to Ohkawa, for an invention entitled xe2x80x9cPlasma Mass Filterxe2x80x9d which is assigned to the same assignee as the present invention. Several aspects of a plasma filter that distinguish it from a plasma centrifuge are noteworthy. In particular, unlike a plasma centrifuge, it is important that a plasma filter operates with a plasma density that is below a collisional density. By definition, and as used herein, a collisional density occurs when the ratio of a cyclotron angular frequency to a collisional frequency is greater than one (i.e. xcfx89c/xcexd greater than 1). Stated differently, in a plasma having a density below its collisional density, there is a high probability that a charged particle will experience at least one orbited rotation before colliding with another charged particle in the plasma. Thus, very much unlike a plasma centrifuge, a plasma filter avoids collisions between the charged particles. Another aspect which distinguishes a plasma filter from a plasma centrifuge is that crossed electric and magnetic fields can be employed in a plasma filter to selectively confine the trajectories of orbiting charged. particles. Specifically, as disclosed for the plasma mass filter by Ohkawa mentioned above, charged particles having a mass-charge ratio below a determinable cut-off mass, Mc, will be confined within a space between the axis of rotation and a radial distance, xe2x80x9ca,xe2x80x9d therefrom. As previously disclosed by Ohkawa, for a cylindrical plasma mass filter chamber, Mc=ea2B2/(8Vctr) wherein there is a radius, xe2x80x9ca,xe2x80x9d a uniform axial magnetic field, xe2x80x9cB,xe2x80x9d and a parabolic radial voltage profile with a central voltage, xe2x80x9cVctr,xe2x80x9d with the wall of the cylinder grounded. The charge on the heavy ion to be separated is xe2x80x9ce.xe2x80x9d
It can happen that it may be desirable, or necessary, to separate a composite material into more than two parts. For example, it may be desirable to separate a nuclear waste into three or more component parts. For example, one part may be a radioactive toxic nuclear component which must be disposed of under most careful circumstances. On the other hand, another part of the composite material may be useful in other different processes. Still another part may be disposable by more ordinary and conventional means.
In light of the above, it is an object of the present invention to provide a multi-mass filter that is capable of separating a multi-species plasma into more than two constituent parts. Another object of the present invention is to provide a multi-mass filter which effectively confines charged particles of different mass-charge ratios to trajectories that direct the charged particles into respectively different regions for segregated collection. Still another object of the present invention is to provide a multi-mass filter that is relatively simple to manufacture, is easy to use, and is comparatively cost effective.
A multi-mass filter for separating particles in accordance with the present invention includes a chamber that defines an axis and has specifically configured crossed electric and magnetic fields (Exc3x97B) inside the chamber. For the present invention, the linearly increasing electric field (E) is generated with a positive voltage Vctr along the chamber axis and is oriented to extend radially therefrom toward a ground at the chamber wall. The magnetic field (B), on the other hand, is generated to extend through the chamber generally parallel to the axis.
With the above in mind, let the term xe2x80x9caz,xe2x80x9d represent a radial distance from the axis at an arbitrary xe2x80x9czxe2x80x9d location on the axis. Similarly, let the term xe2x80x9cBzxe2x80x9d represent a magnetic field strength at the same arbitrary xe2x80x9czxe2x80x9d location on the axis. With xe2x80x9cexe2x80x9d representing a positive ion charge, an expression for cut-off mass becomes Mcz=eaz2Bz2/(8Vctr) assuming a quadratic dependence of voltage with a radius between 0 and a2 and the voltage at the wall is zero since the wall is grounded. As can be shown mathematically for the Mcz, expression, particles that have mass-charge ratios below Mcz, are confined by the crossed electric and magnetic fields inside the chamber between the axis and a radial distance az, from the axis. On the other hand, particles that have mass-charge ratios above Mcz, will be ejected beyond the radial distance az from the axis. As intended for the present invention, a multi-species plasma is introduced into the chamber to interact with the crossed electric and magnetic fields under conditions which allow the particles to orbit around the chamber axis. Specifically, for purposes of the present invention it is contemplated that the multi-species plasma will include particles of relatively low mass-charge ratio (M1), particles of intermediate mass-charge ratio (M2), and particles of relatively high mass-charge ratio (M3). Further, it is contemplated that the multi-species plasma will have a density inside the chamber that is less than a predetermined collisional density. For the present invention, collisional density is defined by considering that all of the particles M1, M2 and M3 will have a collision frequency xcexdcol, inside the chamber. The particles will also have their respective cyclotron frequencies xcfx89m1, xcfx89m2 and xcfx89m3 in response to the crossed electric and magnetic fields (Exc3x97B). Thus, as defined herein, a collisional density occurs whenever xcfx89m1 greater than xcfx89m2 greater than xcfx89m3 greater than Vcol. Stated differently, the predetermined collisional density is defined when a ratio between xcfx89m3 and the collision frequency is greater than one (i.e. xcfx89m3/xcexdcol  greater than 1) and, preferably, much greater than one.
It is a consequence of the present invention that the crossed electric and magnetic fields (Exc3x97B) are created to establish respective first trajectories for each of the particles (M1), second trajectories for each of the particles (M2), and third trajectories for each of the particles (M3). Further, the crossed electric and magnetic fields (Exc3x97B) will also respectively direct each of the particles M1, M2 and M3 along their respective trajectories into respective first, second and third regions to thereby separate the particles (M1, M2 and M3) according to mass-charge ratio.
For one embodiment of the present invention, the magnetic field (B) will vary along the axis. For this embodiment, both the chamber and the magnetic field, B, are configured to maintain the conservation of magnetic flux through the chamber along the axis of the chamber. Specifically, in this embodiment, the chamber wall is distanced farther from the axis in a direction along the axis that will be taken by the multi-species plasma as it transits through the chamber. For there to be a conservation of magnetic flux, however, the term xe2x80x9caz2Bzxe2x80x9d must remain substantially constant in the expression for Mcz. Thus, due to the changes in the cross section of the chamber for this embodiment (i.e. change in xe2x80x9cazxe2x80x9d), the magnetic field Bz, must also be varied. For the present invention, this can be accomplished using magnetic coils that are positioned in planes substantially perpendicular to the axis to surround the chamber. These coils can then be controlled to establish the requisite magnetic field strengths along the axis. In accordance with the present invention, in order for az2Bz to remain constant, as xe2x80x9caz,xe2x80x9d increases, Bz will decrease. Thus, for this embodiment, particles M3 that are greater than Mc3will be ejected into the third region, particles M2 that are greater than Mc2 will be ejected into the second region (where a2 greater than a3 and B2  less than B3) and, finally, the particles M1 will be ejected into the first region (where a1 greater than a2 and B1  less than B2).
For another embodiment of the present invention, the magnetic field (B) in the chamber is maintained so as to be substantially constant along the axis. The electric field (E), however, is established with a particular configuration. Specifically, the electrical field increases linearly at a first rate in a radial direction outwardly from the axis. This first rate of increase occurs through a radial distance a2 and defines the first region. It also establishes a cut-off mass Mc2=er22B2/(8*(Vctxe2x88x92V2)) where V2 is the voltage at a2 (r2) so that M3 and M2, which are both greater than Mc2, will be ejected from the first region. At the radial distance a2 (r2) from the axis, however, the electrical field is caused to decrease, and then linearly increase radially outward at a second, slower rate. Between a2 (r2) and a radial distance a3 (r3), this second, slower rate of increase in the electrical field establishes a cut-off mass Mc3=e(r32xe2x88x92r22)B2/(8*V2) where V3 is the voltage at a3 (r3) and is generally zero. Because M3 is greater than Mc3 and M2 is less than Mc3, particles M3, but not particles M2, will be ejected from the second region into the third region. For this embodiment, the third region is preferably the wall of the chamber. The first and second regions, however, extend axially from the chamber. As contemplated by the present invention, the particular configuration for the electric field (E) in this embodiment can be established using either concentric electrode rings, or spiral electrodes, which are positioned in planes that are oriented substantially perpendicular to the axis.